1. Technical Field
The present invention relates to an electro-optical device, to a method of driving an electro-optical device, and to an electronic apparatus.
2. Related Art
Electro-optical devices, such as liquid crystal devices for displaying images using liquid crystal have been known in recent years. For example, the electro-optical device has the following structure.
FIG. 11 is a plan view illustrating an electro-optical device 1 according to the related art.
The electro-optical device 1 includes a liquid crystal panel AA, a scanning line driving circuit 101, a data line driving circuit 102, and a common line driving circuit 103.
The liquid crystal panel AA includes an element substrate 100 having thin film transistors (hereinafter, referred to as TFTs) 151, serving as switching elements, arranged in a matrix thereon, a counter substrate arranged opposite to the element substrate 100, and liquid crystal interposed between the element substrate 100 and the counter substrate.
The scanning line driving circuit 101 and the data line driving circuit 102 are formed on the element substrate 100 of the liquid crystal panel AA.
The element substrate 100 has a plurality of scanning lines 110 which are provided at predetermined intervals, a plurality of data lines 120 which are provided at predetermined intervals so as to be substantially orthogonal to the scanning lines 110, and a plurality of common lines 130 which are alternately provided so as to be substantially parallel to the plurality of scanning lines 110 formed thereon.
Further, pixel circuits 150 are provided so as to correspond to intersections of the scanning lines 110 and the data lines 120. Each of the pixel circuits 150 is composed of the TFT 151, a pixel electrode 155, and a storage capacitor 153 having one end connected to the pixel electrode 155 and the other end connected to the common line 130.
In each pixel circuit, a gate of the TFT 151 is connected to the scanning line 110, and a source of the TFT 151 is connected to the data line 120. In addition, a drain of the TFT 151 is connected to the pixel electrode 155 and the storage capacitor 153. In this way, the TFT 151 connects or disconnects the data line 120 to or from the pixel electrode 155 and the storage capacitor 153, in response to a control signal transmitted through the scanning line 110.
The counter substrate includes a common electrode 156 arranged opposite to the pixel electrodes 155. The common electrode 156 is connected to the common lines 130 by opposite connecting portions 105 provided at four corners of the element substrate 100 and common wiring lines 106 for connecting these opposite connecting portions 105.
The common line driving circuit 103 supplies to the common lines 130 a control signal having a first potential or a second potential higher than the first potential.
The data line driving circuit 102 supplies image signals to the data lines 120 at a potential higher than the first potential when the potential of the common electrode 156 is the first potential, and supplies the image signals to the data lines 120 at a potential lower than the second potential when the potential of the common electrode 156 is the second potential.
The scanning line driving circuit 101 supplies control signals for turning on or off the TFTs 151 to the scanning lines 110.
In the electro-optical device 1, the common line driving circuit 103 inverts the potential of the common electrode 156 between the first potential and the second potential which is higher than the first potential (hereinafter, referred to as common inversion driving). When the potential of the common electrode 156 is the first potential, the data line driving circuit 102 supplies the image signals to the data lines 120 at a potential higher than the first potential (hereinafter, referred to as positive writing). When the potential of the common electrode 156 is the second potential, the data line driving circuit 102 supplies the image signals to the data lines 120 at a potential lower than the second potential (hereinafter, referred to as negative writing).
FIG. 12 is a timing chart of the electro-optical device 1 according to the related art at the time of positive writing. FIG. 13 is a timing chart of the electro-optical device 1 according to the related art at the time of negative writing. FIGS. 12 and 13 show a structure in which writing is performed at the same grayscale level. In FIGS. 12 and 13, characters VCOM indicate the potential of the common electrode 156, and characters SOURCE indicate the potential of the data line 120. In addition, characters GATE indicate the potential of the scanning line 110, and characters PIX indicate the potential of the pixel electrode 155.
As shown in FIG. 12, in the positive writing, first, the common line driving circuit 103 supplies a control signal for allowing the potential VCOM of the common electrode 156 to turn to a first potential VCL, and the scanning line driving circuit 101 supplies a control signal for allowing the potential GATE of the scanning lines 110 to turn to a potential VGH, thereby turning on TFTs 151.
Next, at a time t1, the data line driving circuit 102 supplies the image signals to raise the potential SOURCE of the data lines 120 from a potential VP1 to a potential VP5, causing the image signals to be written onto the pixel electrodes 155 and the storage capacitors 153 through the TFTs 151.
Next, at a time t2, the scanning line driving circuit 101 turns on the TFTs 151. Then, a difference between the first potential VCL of the common electrode 156 and the potential VP5 written onto the pixel electrodes 155 is applied to the liquid crystal, and is then held by the storage capacitors 153.
Subsequently, at a time t3, the common line driving circuit 130 raises the potential VCOM of the common electrode 156 from the first potential VCL to a second potential VCH. Then, the potential PIX of the pixel electrodes 155 rises up to a potential VP2 together with the potential of the common electrode 156, with a difference between the potential PIX of the pixel electrodes 155 and the potential VCOM of the common electrode 156 kept constant. At that time, the data lines 120 are disconnected from the pixel electrodes 155 by the TFTs 151, but are capacitively connected to the common lines 130, which causes the potential SOURCE of the data lines 120 to rise up to the potential VP2 which is equal to the potential PIX of the pixel electrodes 155.
Next, in the negative writing, as shown in FIG. 13, first, the common line driving circuit 103 supplies a control signal for allowing the potential VCOM of the common electrode 156 to turn to the second potential VCH, and the scanning line driving circuit 101 supplies a control signal for allowing the potential GATE of the scanning lines 110 to turn to the potential VGH, thereby turning on TFTs 151.
Next, at a time t5, the data line driving circuit 102 supplies the image signals to lower the potential SOURCE of the data lines 120 from the potential VP2 to a potential VP6, causing the image signals to be written onto the pixel electrodes 155 and the storage capacitors 153 through the TFTs 151.
Next, at a time t6, the scanning line driving circuit 101 turns off the TFTs 151. Then, a difference between the second potential VCH of the common electrode 156 and the potential VP6 written onto the pixel electrodes 155 is applied to the liquid crystal, and is then held by the storage capacitors 153.
Subsequently, at a time t7, the common line driving circuit 130 lowers the potential VCOM of the common electrode 156 from the second potential VCH to the first potential VCL. Then, the potential PIX of the pixel electrodes 155 is lowered to a potential VP1 together with the potential of the common electrode 156, with a difference between the potential PIX of the pixel electrodes 155 and the potential VCOM of the common electrode 156 kept constant. At that time, the data lines 120 are disconnected from the pixel electrodes 155 by the TFTs 151, but are capacitively connected to the common lines 130, causing the potential SOURCE of the data lines 120 to be lowered to the potential VP1 which is equal to the potential PIX of the pixel electrodes 155.
According to the electro-optical device 1, it is possible to prevent the liquid crystal screen of the liquid crystal panel AA from being burnt.
However, in the conventional electro-optical device 1, when the potential SOURCE of the data line 120 is raised from the potential VP1 to the potential VP5 and when the potential SOURCE of the data line 120 is lowered from the potential VP2 to the potential VP6, a large amount of power is consumed. In addition, it takes a long time to write the image signals onto the pixel electrodes 155.
Further, when the positive writing is performed, capacitive coupling occurs between the data line 120 and the common line 130. Therefore, when the potential VCOM of the common electrode 156 rises from the first potential VCL to the second potential VCH, the potential SOURCE of the data line 120 rises up to the potential VP2 which is equal to the potential PIX of the pixel electrode 155. On the other hand, when the negative writing is performed, capacitive coupling occurs between the data line 120 and the common line 130. Therefore, when the potential VCOM of the common electrode 156 is lowered from the second potential VCH to the first potential VCL, the potential SOURCE of the data line 120 is lowered to the potential VP1 which is equal to the potential PIX of the pixel electrode 155. As a result, an excessively high voltage is applied to the data line driving circuit 102, causing the data line driving circuit 102 to be damaged.
In view of the above-mentioned problems, there has been proposed an electro-optical device including a pre-charge circuit having a large capacitance (for example, see JP-A-2004-354758). In addition, there has been suggested an electro-optical device including a pre-charge circuit for supplying a driving voltage to the data lines (for example, see JP-A-2004-191536).
In the electro-optical device including the pre-charge circuit disclosed in JP-A-2004-354758, when the potential of the common electrode rises in the positive writing, a rise of the potential of the data line to a potential equal to the potential of the pixel electrode is prevented by moving charges between the data line and the pre-charge circuit having a large capacitance. On the other hand, when the potential of the common electrode is lowered in the negative writing, the lowering of the potential of the data line to a potential equal to the potential of the pixel electrode is prevented by moving charges between the data line and the pre-charge circuit having a large capacitance.
In this way, it is possible to prevent the potential of the data line from being raised or lowered without supplying the driving voltage to the data line, and thus to reduce power consumption. In addition, it is possible to prevent the data line driving circuit from being damaged.
In the electro-optical device including the pre-charge circuit disclosed in JP-A-2004-191536, before the potential of the common electrode is raised in the positive writing, the pre-charge circuit supplies the driving voltage to raise the potential of the data line up to a potential equal to the potential of the common electrode. In addition, before the potential of the common electrode is lowered in the negative writing, the pre-charge circuit supplies the driving voltage to lower the potential of the data line to the potential equal to the potential of the common electrode.
In this way, it is possible to reduce a difference in potential before and after the image signals are written by supplying the driving voltage and thus to shorten the time required for writing the image signals to the pixel electrodes.
However, in recent years, portable apparatuses, such as cellular phones provided with electro-optical devices, have come into widespread use. The electro-optical device has a problem in that power consumption increases with the progress of the functions thereof. Therefore, there is a strong demand for an electro-optical device capable of reducing power consumption. However, the electro-optical device disclosed in JP-A-2004-354758 does not sufficiently meet such a demand. In addition, in the electro-optical device disclosed in JP-A-2004-191536, power consumption is reduced when the image signals are written onto the data lines, but the pre-charge circuit consumes power. Therefore, it is difficult to reduce overall power consumption.